Most large-scale heat exchangers, such as the air-cooled condensers used in binary-cycle geothermal power plants, require the use of finned tubes in order to increase the heat transfer surface area. A finned tube in a heat exchanger is generally comprised of a tube with a series of fins extending from the outer surface of the tube along its length. Such fins may be plate-type individual fins or wound in a spiral-type configuration along the length of the tube. In a condenser such as an air-cooled condenser, coolant such as air is typically forced through several rows (or a “bundle”) of long, individually-finned tubes by large induced-draft fans or the like. The condenser units in a power plant can be very large and represent a significant percentage of the overall capital cost of the plant. In addition, the power required to operate the fans typically represents a significant parasitic house load, thereby reducing the net power production of the power plant. Therefore, it would be generally desirable to increase the heat transfer performance of the finned tubes without significantly increasing the cost of the condenser or the power required to operate the fans.
Generating counter-rotating longitudinal vortices in the fluid flow path along the finned tube periphery results in a more efficient exchange of heat. This is due at least in part to the fact that longitudinal vortices disrupt boundary layer formation and mix the fluid (e.g., air) stream near the fin and tube surfaces with the main fluid flow stream. Certain longitudinal vortices, called “horseshoe vortices”, are generated naturally in finned tube heat exchanger passages by the interaction of the fluid flow with the curved surface of a heat exchanger tube. The heat transfer performance of finned tubes can be further improved by generating additional longitudinal vortices, which can be created through the use of vortex generators on the individual fins.
Vortex generators may be comprised of a series of winglets mounted on or punched into the fin surfaces. Depending on the shape of the winglets and the position of the winglets on the fins, heat transfer performance can be significantly improved with a minimal increase in pressure drop along the finned tube.